Lavatory, not for all, in use — then free (5)
I believe the answer is:
loose
'free' is the definition.
(synonyms)
'lavatory not for' is the wordplay.
'lavatory' becomes 'loo' (loo is a kind of lavatory).
'not for' becomes 'se' (I can't justify this - if you can you should give a lot more credence to this answer).
'loo'+'se'='LOOSE'
'all in use then' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for loose that I've seen before include "Not fitting tightly" , "Unattached" , "Relaxed; set free" , "Unconfined" , "easy with one's favours" .)